Ever since the early 19th century, scientists have known that an electric potential can be generated by simply driving an ionic liquid -- fluids with charged ions in it, like water or sodium chloride solution -- through channels or holes under a pressure gradient. The latest in hydroelectric power involves generating voltage by immersing carbon nanotubes in flowing liquids. But scientists have never quite figured out exactly how this happens -- and generating electricity without a pressure gradient remains a challenge.

They experimented with a wide range of various parameters, and they found that the resulting electric potential is proportional to the velocity (of the dragged droplet) and the number of droplets. Other factors they took into account: concentration and ionic species of the drop, as well as the number of graphene layers. Their conclusion: voltage increases when multiple droplets of the same size are quickly dragged all at once over monolayer graphene.

They found that a “pseudocapacitor” (for storing the electric charge) is formed at the interface of the droplet and graphene, and that it’s driven by the moving droplet, charging and discharging at the front and rear of the drop. When the drop is still, the charge is equally distributed on either side; but when the drop is dragged, the charge distribution becomes unbalanced. The electrons from graphene are absorbed at one end and “desorbed” at the other. The difference gives rise to an electric potential on one side of the drop, generating a voltage across its length.

To demonstrate the potential of this electrokinetic phenomenon, they scaled it up and created an energy-harvesting device. A copper chloride drop on a tilted graphene surface generated about 30 millivolts.